Reasoning GP · Algorithm · 1-page summary

Hyponatraemia in adults

For: adults with Na⁺ <135 mmol/L, in hours or OOH. Not for: pregnancy or peripartum (NICE NG235, specialist advice); children; hospital correction (full algorithm, step 7).

Reviewed October 2026 · next review April 2027

Soc Endocrinology 2022European guideline 2014NICE NG243NG12CG174

Core rule: symptoms and the speed of the fall drive urgency, not the sodium alone. Urine osmolality, urine sodium (sent before treatment) and volume status name the cause; always review drugs. Correction limits are maximums, not targets: never more than 10 mmol/L in the first 24 h, then 8 per further 24 h (slower still if high risk) — osmotic demyelination is irreversible.

0Stop — call 999 now — emergency admission at any sodium level

Any symptom attributable to it: seizure, GCS ≤14, drowsiness, confusion, vomiting, respiratory distress, unequal pupils; also headache or nausea without vomiting → 999
Na⁺ <120 mmol/L with neurological symptoms, acute onset or a rapidly falling sodium; acute fall (<48h) with any neurology; symptomatic postoperative (within 3–5 days), exercise, MDMA, bowel-preparation or water-intoxication hyponatraemia → 999
Shock: HR >120, SBP <90, postural collapse → 999. Suspected adrenal crisis (low Na⁺ with hypotension, vomiting, collapse or raised K⁺; after stopping long-term steroids) → 999
While waiting: adrenal crisis → IM or IV hydrocortisone now if trained and authorised; never wait for cortisol. 3% saline is hospital only. Do not delay transfer for tests.

1Triage by sodium — if asymptomatic

Na⁺ mmol/L	Action
<120	Same-day hospital or acute medical assessment, even if well.
120–124	Same-day hospital assessment, even if well, stable and chronic (NICE CKS).
125–129	Same-day face to face, bloods before treatment, specialist advice; repeat Na⁺ within 24–48 hours.
130–134	Primary care if well, stable, chronic, clear reversible cause, repeat plan. Onset unknown = chronic.

2Screen — one row is enough

Adrenal insufficiency? (postural hypotension, pigmentation, K⁺ raised) → Same-day admission
Hypervolaemic (HF, cirrhosis, nephrotic); with hypokalaemia, malnutrition, alcohol dependence or liver disease; worsening despite stopping the drug; poor intake → Same-day admission
New SIADH, no obvious cause → Urgent malignancy screen: chest X-ray (SCLC); CT if high suspicion.
NG12, 40+: ≥2 of cough, fatigue, breathlessness, chest pain, weight or appetite loss (≥1 if ever smoked) → urgent chest X-ray; unexplained haemoptysis or suggestive X-ray → suspected cancer pathway. Not for hyponatraemia alone.

3Assess — face to face if Na⁺ <130

☐ GCS, orientation, gait; lying and standing BP (drop >20 mmHg systolic = hypovolaemia) ☐ JVP, oedema, ascites, crackles, pigmentation, nodes ☐ U&E, serum osmolality, glucose; spot urine Na⁺ and osmolality, before IV fluids; TFTs; 8–9 am cortisol if adrenal insufficiency possible; LFTs; lipids, protein if pseudo ☐ All medicines

4Classify — osmolalities first; examination refines

Serum osmolality 275–295 → pseudohyponatraemia (lipids, paraprotein); >295 → hyperglycaemia.
Urine osmolality ≤100 → primary polydipsia, low solute (beer potomania), excess water.
Urine osm >100, urine Na⁺ ≤30 → reduced effective volume: GI or skin losses (dry, postural drop), or HF, cirrhosis, nephrotic (oedema).
Urine Na⁺ >30 → SIAD, diuretics, Addison's, renal salt loss (unreliable with diuretics, CKD, saline). SIADH only once hypothyroidism, adrenal insufficiency and diuretics are excluded; find its cause (SCLC, CNS, chest, drugs).
Drugs — thiazides (commonest in elderly), SSRIs/SNRIs, carbamazepine, tricyclics, PPIs, NSAIDs, opioids, antipsychotics.
Uncertain — say so; specialist advice; never "SIADH" by default.

5Treat today — mild, stable, well

Remove the cause: stop the offending drug if safe; recheck Na⁺ promptly. Overt hypothyroidism → thyroid pathway.
Hypovolaemic, stable, drinking: oral rehydration, stop thiazide — never fluid-restrict.
SIADH: individualised fluid restriction, all drinks counted (no generic 500 mL/day without specialist input); review at 1 week. Hypervolaemic: specialist-led; no reflex IV fluids.
Specialist only: urea, demeclocycline, tolvaptan — never start in primary care. Doses: NICE/SPC, BNF, local formulary.

6Refer routinely

Endocrinology: confirmed SIADH, stable 130–134. Nephrology: CKD-related, refractory. Hepatology: cirrhosis. Falls service if Na⁺ <130 with falls. After discharge: recheck per plan, within 1 week for many.

7Never reassure by

The patient seeming well below 120 — same-day hospital assessment regardless.
A "mild" number in a patient with symptoms or a fast fall.
Low Na⁺ plus raised K⁺ each looking mild — think adrenal crisis.
"SIADH" without TFTs, cortisol, urine tests and a drug review.
Fluid restriction in a hypovolaemic patient, or saline in a hypervolaemic one.
A rapidly rising sodium — over-correction causes osmotic demyelination.

8Safety-net — say it, then document it

999 “Call 999 for a fit, collapse or fainting, unusual drowsiness or being hard to wake, new or worsening confusion, a severe headache, vomiting, or breathing difficulty.” Same day: “Contact us or 111 today if you feel sick, dizzy on standing, unsteady or fall, get muscle cramps, cannot keep up with fluids, or are very thirsty with little urine. Attend the repeat blood test even if well.”

Remote or OOH: Na⁺ <130 needs face to face — volume status cannot be judged by phone. Convert to face to face or emergency if unable to attend, vomiting, diarrhoea or poor intake, a relative reports confusion, or no one can act on an out-of-hours result.

✓Safe to manage in primary care only if — all five, documented

Na⁺ generally ≥130, stable, chronic; no block 0–2 row No neurological symptoms; volume status examined Urine Na⁺ and osmolality, TFTs, cortisol sent Clear reversible cause; drug stopped if safe Repeat date, thresholds and safety-net recorded

Summary of the full Reasoning GP hyponatraemia algorithm, which holds the detail, examination, treatment pathway and evidence. Sources: Society for Endocrinology emergency guidance (2022); European clinical practice guideline on hyponatraemia (2014); NICE NG243, NG12 (updated April 2026), CG174; local formulary. Clinical reference — follow local acute-transfer, renal and prescribing policies and check doses: NICE/SPC, BNF.